Processing of antigens by antigen-presenting cells The processing of antigens by antigen-presenting cells (APC) takes place by two different routes. Antigens occurring inside the cell are presented by MHC I (major histocompatibility complex class I, MHC class I) molecules on the cell surface, whereas extracellular antigens are presented by MHC II (major histocompatibility complex class II, MHC class II) molecules on the cell surface. Both mechanisms initiate an immune reaction by the host to the antigen. The route taken by the antigen from uptake into the cell until presentation on the cell surface in the form of an MHC II-antigen complex proceeds via various cell organelles, inter alia via the endoplasmic reticulum, the Golgi apparatus, the trans-Golgi network, lysosomes, endosomes and via MHC class II compartments (MIIC). The MIICs play an important part in the MHC II-mediated antigen presentation. In these organelles of the cell, the MHC II molecules are loaded with low molecular weight antigens or with proteolytic fragments of proteins. In this process, the invariant chain (also called MHC II gamma chain or Ii) which is initially bound to the MHC II molecule undergoes proteolytic degradation, and the antigen is bound to the MHC II molecule under the regulation of various proteins which bind directly or indirectly to MHC II [1]. These regulatory molecules include inter alia HLA-DM, HLA-DO, LAMP-1, LAMP-2, CD63, CD83, etc. The exact function of these proteins is in part unexplained as yet, but many of them have signal sequences which promote their transport to the lysosomes, to the endosomes, to the trans-Golgi network, to the MIICs etc. [2-4]. A number of proteases are involved in the proteolytic reactions necessary so that the antigen can be presented on MHC II molecules. The proteases present in MIICs include inter alia various members of the cathepsin family such as, for example, cathepsin S and cathepsin L [1].